In recent years, lithium cells, which can be made in ultra thin forms and readily reduced in size, are being actively developed as the cells for use within portable terminals such as laptop computers and mobile phones, video cameras, and satellites and the like. In terms of the packing material used within these types of lithium cells, rather than the metal cases used as the packing material for conventional cells, multilayer films (such as a configuration including a heat-resistant base material layer/an aluminum foil layer/and a thermal adhesive film layer) formed in the shape of a pouch are now frequently being used, as they are lightweight and allow the shape of the cell to be selected freely.
Lithium cells contain, as cell contents, a positive electrode material, a negative electrode material, and either an electrolyte solution prepared by dissolving an electrolyte (a lithium salt) in an aprotic solvent having a penetrative ability such as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate or ethyl methyl carbonate, or an electrolyte layer formed from a polymer gel impregnated with the above electrolyte solution. However, if the solvent having a penetrative ability penetrates through the thermal adhesive film layer that acts as a sealant, then the strength of the lamination between the aluminum foil layer and the thermal adhesive film layer tends to deteriorate, and eventually leads to leakage of the electrolyte solution.
Furthermore, as the lithium salt that acts as the electrolyte, LiPF6 or LiBF4 or the like is used. Because these salts generate hydrofluoric acid via a hydrolysis reaction with moisture, they can cause corrosion of metal surfaces or deterioration in the lamination strength between the various layers of a multilayer film. By using an aluminum foil, the penetration of moisture from the surface of the packing material can be substantially blocked. However, in the lithium cell packing material, the multilayer film typically has a construction that is bonded together by heat sealing, meaning that hydrolysis of the lithium salt caused by moisture that penetrates via the edge face of the seal provided by the thermal adhesive film layer that functions as the sealant remains a concern. Accordingly, strengthening the interlayer adhesive strength between the aluminum foil and the thermal adhesive film layer in order to improve the durability (the electrolyte solution resistance and hydrofluoric acid resistance) of the cell contents is necessary.
Moreover, lithium cells are widely used in portable mobile phones, and the usage environment may sometimes reach very high temperatures of 60 to 70° C., for example, inside a vehicle in the middle of summer. A packing material for a lithium cell that exhibits favorable resistance to the electrolyte solution even under these types of high-temperature conditions has been keenly sought.
As a result, various methods are being investigated to inhibit the delamination that occurs between the aluminum foil layer and the thermal adhesive film layer due to the effects of the electrolyte solution or the hydrofluoric acid generated by hydrolysis of the lithium salt that functions as the electrolyte (see Patent Documents 1 to 4).
Patent Documents 1 to 3 disclose packing materials for lithium cells prepared by techniques such as extrusion lamination or thermal lamination, which do not undergo delamination even under the effects of the electrolyte solution or hydrofluoric acid. Patent Document 4 discloses a technique of improving the urethane-based adhesive used in a dry lamination method. This technique yields a urethane-based adhesive with superior electrolyte solution resistance, meaning a packing material that inhibits delamination can be obtained even using a dry lamination method.
However, in recent years, the functions demanded of the lithium cell packing materials used for packaging lithium cells have continued to increase. One example of a function now required of the packing material for a lithium cell is water resistance. However, as described above, because hydrofluoric acid is generated by hydrolysis of the lithium salt that functions as the electrolyte, evaluations that use water have generally not been included within the methods used for evaluating the lithium cell packing material. However, amongst the various environments in which the lithium cell may be used, accidents such as a situation where a mobile telephone is accidentally dropped into water can be readily conceived. In such cases, there is a possibility that the lack of water resistance may cause delamination, or that the increase in hydrofluoric acid production caused by the excessive absorption of moisture may cause corrosion of the aluminum foil, resulting in delamination. Accordingly, further improvements in both the water resistance and hydrofluoric acid resistance are desirable.
For these types of reasons, the necessity of evaluating water resistance as one of the methods used for evaluating lithium cell packing materials is gradually becoming more accepted. Typically, when performing an electrolyte solution evaluation for the lithium cell packing material, a packing material sample cut into a strip is dipped in the electrolyte solution at a temperature of 85° C. In order to minimise handling and also include an evaluation of water resistance, a method has been proposed in which the strip sample is washed with water following the electrolyte solution dipping treatment, and subsequently subjected to a water dipping treatment. Moreover, an accelerated test is also sometimes used, in which the dipping treatment at 85° C. is conducted using an electrolyte solution to which several thousand ppm of water has already been added, thereby performing the evaluation under conditions in which hydrofluoric acid already exists.
However, with the lithium cell packing materials disclosed in Patent Documents 1 to 3, the water resistance is not entirely satisfactory. Further, the packing material disclosed in Patent Document 4 also suffers from poor water resistance.
Furthermore, it is thought that lithium cells will not only be useful in miniaturized applications such as the types of portable mobile phones and the like mentioned above, but will also become increasingly important in large-scale applications such as cells for motor vehicles or the like. Motor vehicle applications, in particular, will require improvements in the electrolyte solution resistance, water resistance and hydrofluoric acid resistance beyond current levels.
The most effective known method of imparting these resistance properties is performing a chemical conversion treatment on the aluminum foil, and one example of this type of chemical conversion treatment is a chromate treatment.
For example, Patent Document 5 discloses a multitude of chromate treatments, including coating type chromate treatments and chromate treatments that employ dipping methods.
Further, in all manner of chemical conversion treatments not limited to chromate treatments, the aluminum foil may be imparted with an etching function so that the aluminum foil and the chemical conversion treatment layer formed by the chemical conversion treatment adopt a graded structure. In order to achieve this effect, any of the various inorganic acids such as hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid and hydrofluoric acid, or salts thereof, may be used as treatment agents.
However, in the type of chromate treatment disclosed in Patent Document 5, the hexavalent chromium used as the main component of the treatment material has been identified as an environmental toxin, and although it exhibits favorable functionality, the material is unattractive from an environmental perspective. As a result, trivalent chromium has become widely used, but achieving the same effect as that observed for hexavalent chromium is difficult, and as long as chromium continues to be used, chromate treatments will remain undesirable from an environmental point of view.
Furthermore, the treatment agents used during chemical conversion treatments often cause corrosion of the coating apparatus, which not only places limitations on the coating apparatus, but also tends to result in deterioration in the operating environment.
Moreover, in order to improve the adhesion of these treatment agents, a dipping treatment in an acid bath or alkali bath, and steps for performing degreasing or etching may be included within the production steps for the cell packing material. However, although these steps are necessary in term of imparting favorable electrolyte solution resistance, the treatment cost is high, and the steps tend to be rate-limiting in terms of the production of the cell packing material, meaning a significant simplification of the production steps is currently required.
[Patent Document 1]
Japanese Unexamined Patent Application, First Publication No. 2001-243928
[Patent Document 2]
Japanese Unexamined Patent Application, First Publication No. 2004-42477
[Patent Document 3]
Japanese Unexamined Patent Application, First Publication No. 2004-142302
[Patent Document 4]
Japanese Unexamined Patent Application, First Publication No. 2002-187233
[Patent Document 5]
Japanese Unexamined Patent Application, First Publication No. 2002-144479